Frequency modulation



M. G. CROSBY FREQUENCY MODULATION Maih 3, 1936.

Filed July 9, 1952 AMPl/F/[H AND 01? 3 Sheets-$het 1 INVENTOR MURRA G. CROSBY BY l m ATTORNIEY March 3, 1936. CROSBY 2,032,403

FREQUENCY MODULATION Filed July' 9, 1952 s Sheets-Sheet 2 DYIVHTEUN Oscummes (WI/Pl //V6 OSCILLHTOZS sou/w INVENTOR MURRAY 6. CROSBY ATTORNEY March 3,1936. M, G, Q BY. 2,032,403

FREQUENCY MODULAT I ON Filed July 9, 1952 s SheetS-v-Sheet a x N INVENTOR N N N MURRAY GCROSBY ATTORNEY Patented Mar. 3, 1936' UNITED STATES PATENT OFFICE FREQUENCY MODULATION of Delaware Application July a, 1932, Serial No. 821,568

13 Claim.

Thisinvention relates to an improved method of and means for imparting to a radio wave or to high frequency oscillations certain characteristics which are representative of a signal to be transmitted in order that said wave when received may be reduced or separated into its components and the characteristics thereof representative of the signal rendered intelligible.

More'in detail, the present invention relates to a inethod of and means for impressing-on a radio wave or high frequency oscillations variations in frequency which are characteristic of a signal to be transmitted.

More in particular, this invention relates to a new and improved frequency modulation transmitter wherein two oscillators, tuned to frequencies on either side of the wave or carrier freqiiency oscillations, are coupled together and held in step by two modulator tubes. The modulator tubes are differentially modulated so that the resultant frequency is frequency modulated between the separate individual frequency of the oscillations produced by each oscillator.

Frequencymodulated transmitters have been known heretofore in the art. In the design of frequency modulated transmitters of the kind known heretofore in the art,it has been the practice to couple the two oscillator tubes directly together by means of capacity or inductive coupling between the grid or plate circuit of the two oscillators. This sort of coupling is very unsatisfactory due to the fact that the oscillators can not be tuned. separately and then coupled together by the capacities and/or inductances since coupling one oscillator to the other changes thetune of both. Consequently, the two oscillators must be separately tuned while they are coupled together and this becomes a cut-and-try system in which the required adjustment is diflicult to obtain.

The above disadvantages are overcome in accordance with the present invention by providing the coupling between the oscillator tubes through vacuum tubes so that there is no reaction of one oscillator upon the other. In a particular form of the present invention this coupling is obtained through the themiionic modulator tubes used for impressing the signal modulations on the carrier wave.

. An apparent advantage to be gained by the use of a transmitter constructed as described in the present invention is that each oscillator may be tune and adjustment of the oscillator circuits.

have been pointed out with particularity in the claims appended hereto.

The nature of the invention, the mode of operation thereof,.and the advantages resulting from the use thereof will be apparent from the detailed description which follows, and therefrom when read in connection with the drawings,

throughout which like reference numerals indicate like parts, and in which:

Fig. 1 shows a circuit arrangement illustrating way of the ground G and cathode a of tube A.

In order that a normal direct current biasing potential may be applied to the control electrode 1, a resistance R is connectedbetween the 'control electrode 1 and the cathode 8. A tank circuit Ill, comprising a variable condenser 'ii and inductance l2, is'connected with the anode ll of the tube A, The lower terminal of the tank circuit i0 is connected, as indicated, to a positive terminal of a source of potential not shown. In order that the radio frequency oscillations generated in tubeA do not pass through the potential source, a condenser 9 is connected between the lower terminal of the tuned circuit I0 and the I0 and ill of oscillation generators A and B are .both inductively coupled to the input circuit it i of a power amplifier and/ or frequency multiplier i8. The oscillations produced by A and B together and modulated in a manner which will ap' pear more in detail hereinafter are impressed on the input circuit it of the frequency amplifier or multiplier i8 and passed from there to radiating circuitlll. The nature of the radio frequency amplifier and/or multiplier and of the-radiating circuit forms no part of the presentinve ion and will not be described in detail here.

The oscillators A and B are exactly similar to each other except for the fact that, if it is as- .sumed that the frequency of the oscillations to be normally produced'is equal to f, the tank circuit iii in the output circuit of the tube A is tuned to a point such that tube A oscillates at a; frequency f+f1 whereas the tuned tank circuit II in the oscillation circuit of B is tuned 'to a pointsuch that the tube B oscillates at a frequency modulators.

f-h. In order that the two oscillators A and B develop oscillations constant in frequency and separated by a. constant different frequency means is provided to lock the two oscillators in electrode 2| is supplied, as indicated, at C from a source not shown. Radio frequency oscillar tions appearing in the input circuit of tube C are prevented from passing through the biasing source by means of a by-pass condenser 24. Potential variations are impressed from inductance |2 on to inductance 23 and on to the input electrodes 2|, 22 of tube C, in which tube they are amplified and relayed and appear on the anode electrode 26. The anode electrode 26 is connected in circuit with an inductance 28 coupled to the inductance 2' in the input circuit of oscillation generator circuit B so that potential oscillations are impressed on the inductance 2' and govern to some extent the frequency oscillations developed by B. Charging potential for the anode 26 of tube C is supplied, as indicated, at B from a source not shown. In a similar manner the output inductance l2 of oscillation gen-- erator B is coupled by way of thermionic modulator tube D and inductance 28 to the inductance 2 in the input circuit of oscillation generator A, in that way governing to a certain extent the frequency of the oscillations developed by A.

The two oscillators A and B when oscillating uninfluenced by each other oscillate at frequencieS f-l-fi and f-f1 respectively. Therefore for tuning and adjusting the arrangement the modulator tubes C and D are switched off.

During operation however the tubes C and D are energized and oscillators A and B mutually influence each other to produce oscillations of a frequency f. This tendency of two oscillators mutually coupled to pull into step and generate a single frequency is inherent, is understood and need not be gone into here.

Applicant has up to this point disclosed a novel arrangement. whereby two distinct high frequency oscillations of constant frequency, and separated by a constant frequency difference, may be developed in two separate oscillators, and means whereby the said oscillators may be caused to influence each other to produce a single mean frequency. These means and the oscillations produced thereby may be utilized in any manner. Applicant has however provided further novel means whereby these two constant fre quencies form the upper and lower limits between which a carrier frequency may be varied in frequency relative to the mean frequency in accordance with the signals which it is wished to transmit on the carrier.

This frequency modulation is accomplished in accordance with-the present invention by differentially modulating the oscillations produced in each of the oscillators. This modulation may be accomplished in any manner. Preferably it is accomplished by applying differentially to the oscillators or to the modulators the modulating signals.

Differential modulation of the oscillations may be accomplished as illustrated in 1 by connecting the screen grid electrode 20 of tube D with one terminal of the secondary winding 3| of a modulation transformer T while the other terminal of the winding 3| is connected, as shown, to the screen grid electrode 30' of modulator tube D. Potential variations at signal frequency are set up in the primary winding, 32 of transformer T in any manner. These signal oscillations may be impressed on the winding 32 from a microphone 34 connected in series with the biasing source 35 with the winding 32. It will be understood that the means 34 may be replaced by keying means or by any other means which will produce potential oscillations at signal frequency. Any radio frequency oscillations appearing on the screen grid electrodes 30 or 30' of tubes C and D are blocked out of transformer and 31 provide a path for such high frequency oscillations between the screen grid electrodes 30 and 30' and the cathodes 22 and 22' respectively.

The operation of the device is as follows:

If modulating potentials are applied to the winding 3| they appear differentially on the screen grid electrodes 3|! and 30' andalter the characteristics of these valves sufficiently to change the amount of coupling between the output of one oscillator circuit and the input of the other differentially, thereby swinging between the limits l+f1 and 1-11 the carrier frequency f. This variation in frequency of the carrier f is directly representative of the character of the signal oscillations impressed on'3l. Consequently, this system produces oscillations of varying frequency, the frequency of which is truly characteristic of the signal to be transmitted.

In a modification two dynatrons serve the purpose of producing two distinct high frequency waves separated by a constant difference and varying at signal frequency the carrier between said limits. The nature of this circuit will be understood by reference to Fig. 2 of the drawings in which A is a dynatron oscillator having its anode I connected through a tank circuit 2 to the positive terminal of a source of potential not lower terminal of tank circuit 2 and the cathodev III of tube A.

The anode of this tube is maintained at a potential less than the potential applied to the screen grid electrode thereof so that a negative.

resistance effect is produced in the anode cathode circuit, thereby insuring .the development of potential variations of a high frequency determined by the natm'e of the tank circuit 2. This renders unnecessary the use of the impedance of the tube 4 between the cathode and control electrode to produce oscillations. This impedance or conductance of the tube is therefore utilized in accordance with the present invention to couple the oscillation generators together to maintain them in step so that together they produce a frequency which is a mean between their individual tunings. In this manner therefore the present modification obviates the necessity of using the separa lations produced in the tank circuit 2 (without coupling to A) connected to the anode of B, are

modulator tubes C and D of the circuit of Fig. 1.

In a "similar manner the electrodes, of thermionic dynatron B are connected with a tank circuit 2' to produce oscillations in the same manner in which oscillations were produced in the output circuit, of A. The oscillations produced by A alone i. e., without being coupled to B, however, as determined by the constants of the oscillation circuit 2, are equal to a frequency ,f+f; where j is the carrienwhereas the frequency of the oscilof a frequency equal to ff1. In order to couple the oscillation circuit 2 of tube A with the control electrode l2 of tube B and inductance l4 variably coupled to the inductance 6 has one terminal connected to the control electrode I 2', of tube B and the other terminal connected to ground. Since the cathode ID of tube B is grounded, the circuit between the control electrode l2 and cathode I is completed by way of the ground. In a similar manner the oscillation circuit 2' of tube B is coupled to the control electrode l2 of tube A as shown. The oscillation circuits 2'and 2' are coupled to the utilization circuit l8 by way of coupling inductances II and M respectively, as shown. When the two tubes are mutually coupled in this manner they will influence each other so that instead of each oscillatory at its distinct frequency they will entrain and cooperate to produce a single mean frequency f.

Up to this point applicant has disclosed a novel arrangement whereby an oscillation variable between two diiferent frequencies, maybe produced and impressed on the utilization circuit.

- In order that the frequency of the carrier produced may be varied between the two limits f+j1 and 1, the oscillation generators A'and B are differentially modulated at signal frequency.

This is accomplished in the same manner in which modulation frequencies were differentially impressed on the modulators C andDof Fig. l. A

75 and B may be supplied by connecting the tap on detailed description thereof is therefore thought unnecessary at this point. It will be noted, how-' ever, that the modulation frequencies appearing in 22 are impressed on the primary winding 28 and from the primary winding 28 to the secondary winding 28, and from there to choking inductance 24 and 24' and to the screen grid electrodes i and I! of tubes A and B. Charging potential of a direct current potential higher than that applied to the anodes i and is applied at a point on 25 to the screen grid electrodes l2 and I2. In operation the oscillations produced at 2 and 2' are varied between frequencies f+f1 and 1-41 at signal frequency and are impressed on the utilization-circuit l8.

In the arrangement of Fig. 3 the oscillation generators A and B are of the screen grid type. These oscillators are connected in substantially the same mannerin which the oscillators A and B are connected in Fig. 1. However, this-arrangement differs from the arrangement of Fig. 1 in that the tubes A and B here are'of the 4 electrode type andthe modulating frequencies are applied to the screengrid electrodes 40 and 40' i of the tubes from a filter circuit FC, which is connected to the secondary winding ll of a transformer 42, the primary winding 42 of which may be connected with a source of modulating frequency, as'indicated. Charging potentials for the screen-grid electrodes 4. and 40' of tubes A 3 the secondary winding 4| to the positive terminal of a direct current source not shown, the negative terminal of which is connected to ground so that the screen'grid energizing circuitis completed byv way of ground to the cathodes 8 and. 8 of the tubes A and B respectively. Of course, potential for charging the screen grid electrodes may be taken from the charging sources 44 and 44" in the anode-circuits of tubes A and B respectively.

Where screen grid tubes are used it is often necessary to provide capacitive coupling between the anode and control grid of the tubes inv addition to the inherent capacitive coupling between said electrodes. This may be accomplished by connecting the anodes I4 and H to the control electrodes I and I of tubes A and B respectively by way of variable capacities C1 and C2. The circuits l0 and I, and I0 and l' are tuned to a point such that each of the tubes A and B normally, by themselves, produce oscillations at frequencies f+| and f"| respectively.

The thermionic coupling means whereby these tubes are mutually coupled to influence each other to produce a single mean frequency j, which is varied between the limits j+| and f-I when signal frequency is .applied to the oscillators A and B, will now be described.

The controlgrid electrode 2| of coupling tube 0 is connected to one terminal of an inductance 23, the other terminal of which is connected to ground by way of a biasing source 44. The cathode 22 of coupling tube C is connected to ground, thereby completing the cathode control grid circuit of tube C. In a similar manner the control grid 2| of tube l) is connected to the cathode 22' by way of a couplinginductance 23' and biasing source 44. Inductances 23 and 23' are inductive ly coupled, as in Fig. 1 w the inductances l2 .and I2 in the output circuits of the oscillation generators A and B respectively. g

. The anode 26 of coupling tube 22- is coupled by way of capacity 46 to the tank circuit I of tube A. The anode 26' of tube D is coupled by way of coupling capacity 46' to the tank circuit 1'.

Charging potentials for the anodes 2G and 26' and for the screen grid electrodes 30 and 30' of tubes C and D respectively are supplied from sources- 48 and 48'. The anode electrodes are charged by way of radio frequencychoke coils 49 and 49' in order to prevent any high frequency os-.

cillations appearing in tank circuits and I from reacting on the coupling tubes C and D.

In operation the oscillations produced by B, the frequency of which is determined in part by the tank circuit III; are impressed by way of inductance 23' on to the control electrode 2| of coupling tube C. These oscillations, amplified and repeated in C, are impressed on to the tank circuit I, connected with the input electrodes of oscillation generator A,- to influence the frequency of the oscillations produced therein by determining to some extent the tune of the input circuit. In a like manner the oscillations normally produced by A'. the frequency of which is deter- 4 g cillatlons at frequencies f+l and !--l respectively, but when coupled together, by way of coupling tubes 0 and D, produce a mean frequency I. 'Here,

also, as in the prior modifications, the modulatquency of the oscillations produced by the two generators A and B is accomplished.

The frequency modulated oscillations appearing in the tank circuits In and III are impressed upon the circuit l6 connected to the input electrodes of a pair of buffer tubes E and F, and from the output circuits of tubes E and F said frequency modulated oscillations may be fed by way of tank circuits 50 and 50 to any utilization circuit, as, for example, to an antenna. These frequency modulated oscillations may be fed by way of a frequency amplitude and/or multiplier l8,to the antenna circuit, as in Fig. 1.

Where a more simplified arrangement is desired the device of Fig. 4 may be used. In this arrangement dynatron oscillators A and B are utilized in circuits somewhat similar to the circuits of Fig. 1. Here, however, mutual coupling between the separate oscillators A and B is provided by coupling the anode l of oscillator A by way of a capacity C4 to the control electrode I! of tube B and the anode I of tube B by way of capacity C5 to the control electrode l2 of tube A. The control grids l2 and I! of the dynatron oscillators A and B are maintained at an operating potential by connecting them to the cathodes ill and I0 by way of resistances R2 and Here, as in the prior modifications, the generators A and B, each, when taken separately, generate a different frequency but when mut ually coupled by way of capacities C4 and C5 together generate a mean frequency. Here, also,

as in-the prior modifications, this mean frequency 1 generated by tubes A and B is caused to oscillate at signal frequerrcy between the limits f+l and.fl by applying modulating potentials from the modulating potential source by way of transformer 42 and filter FC to the screen grid electrodes l5and I5 of tubes A and B. Charging potential for the screen grid electrodes l5 and I5 is supplied by connecting the tap on winding 4| to either of the sources 44 or 44 as shown.

The frequency modulated oscillations are impressed on the circuit 16 from which they may be utilized in any manner. For example,they may be coupled by way of coupling tubes similar to E and F of Fig. 3 and amplifiers or frequency modulators of unit l8 of Fig. lto an antenna circuit, from which they may be radiated.-

It will be understood that the above embodiments of my invention are given for purposesof illustration and are not intended to limit the same since it will be obvious that many modifications may be made therein without departing from the spirit oLthe invention and marking out in the claims appended hereto. For example, it will be' obvious to anyone skilled in the art that plate modulation or grid. modulation, or any type of modulation known, might be utilized in place of the screen grid modulation shown for the purpose of varying the intensities of the oscillations or the amount of coupling of one oscillator to the other. All of the frequency modulators described above may .be used to produce phase modulated carriers. This may be accomplished'by the use of audio frequency correction circuits as illustrated in United States application Serial No. 608,383, 5

filed April 30, 1932.

Having thus described my invention and the operation thereof, what I claim is:

1. Signalling means including signal generating and signal modulating means comprising, a lo pair of thermionic oscillation generators each-' having input and output circuits normally tuned to produce oscillations of different predetermined frequencies, thermionic means for coupling theoutput circuit of each of said generators to the input circuit of the other of said generators to entrain said generators to produce a single frequency which is a mean between thefrequencies normally produced;by said generators, a source of signal potentials, and means connectingsaid source of signal frequencies to said generators for varying at signal frequency the internal impedance of the tubes in each of said generators.

2. Signalling means including a signal generating and signal modulating means comprising a pair of thermionic oscillation generators each having input and output circuits normally tuned to produce oscillations of different predetermined frequencies, thermionic means for coupling the" output circuit of each of said generators to the input circuit of the other of said generators to entrain said generators to produce a single frequency which is a mean between the frequencies normally produced by said generators, a source .of signal potentials, and means for varying at signal frequency the internal impedance of the tubes in each of said generators.

3. Means for producing radio waves comprising an oscillation generator tuned to oscillate at 1 a frequency above the normal'frequency of the 40 oscillations to be produced, an oscillation generator tuned to oscillate at a frequency below the frequency of the oscillations. to be transmitted, and thermionic coupling means between the output circuit of each of said generators and the in- 5 put circuit of the other generator to entrain said generators to operate at the same frequency.

4. Means for producing carrier waves comprising an oscillation generator having an input circuit and an output circuit normally tuned to oscillate at a frequency above the frequency of the carrier to be produced, a second oscillation generator having an input circuit and an output circuit tuned to oscillate at a frequency below the normal frequency of the carrier to be produced,

-. and a thermionic tube coupling the outputcircult of each of said oscillation generators to the input circuit of the other of the oscillation gen-- erators to entrain the generators to operate at the same frequency. 6 5. Transmitting means comprising an oscillation generator having an input circuit and an output circuit normally tuned for oscillation at a frequency above the frequency of the carrier to be produced, a second oscillation generator having an input circuit and an output circuit tuned for oscillation at a frequency below the normal frequency of the oscillations to be produced, a thermionic tube couplingthe output circuit of each ofsaid oscillation generators to the input circuit of the otherof said oscillation generators to entrain the generators to operate at the same frequency, and means for varying the conductivity of said coupling tubes at signal frequency.

' and a utilization circuit coupled to the anode cirwaves comprising an oscillation generator having a tuned input circuit and an output circuit tuned to oscillate at a frequency above the normal frequency of the oscillations to be transmitted, a second oscillation generator having a tuned input circuit and an output circuit tuned to oscillate at a frequency below the frequency of the oscillations to be transmitted, a thermionic coupling means between the output circuit of each of said generators and the input circuit of the other generator to entrain the generators to operate at a common frequency, a source of signal potentials and means connected between said source and said coupling means for varying the impedance of said coupling means at signal frequency.

'7. Carrier frequency producing and modulating means comprising, a pair of thermionic tubes of the screen grid type, each having their anodes connected in tuned circuits and their control electrodes connected in circuits, said circuits being coupled to produce sustained oscillations, said anode circuits being tuned to different frequencies, a thermionic tube coupling the anode circuit of one of said tubes to the control electrode circuit of the other of said tubes, a second thermionic tube coupling the anode circuit of said last named tube to the control electrode circuit of said first named tube, a source of modulating potentials connected in phase opposition to the screen grid electrodes of said first named pair of tubes,

cults of both of said oscillator tubes.

8. Carrier frequency producing and modulating means comprising, a pair of thermionic tubes of the screen grid type, each having their anodes connected in tuned circuits and their control electrodes connected in tuned circuits, said anode circuits being tuned to different frequencies, a thermionic tube of the screen grid type coupling-the anode circuit of one of said tubes to the control electrode circuit of the-other of said tubes, a second thermionic tube of the screen grid type coupling the anode circuit of said last named tube to the control electrode circuit of said first named tube, a source of modulating potentials connected in phase opposition to the screen grid electrodes of said last named tubes, and a utilization circuit coupled tothe anode circuits of both of said oscillator tubes.

9. A device for producing carrier frequency.

oscillations and modulating the same in frequency including, a pair of thermionic tubes,

each having a cathode, anode, control electrode and screen grid electrode, oscillation circuits including reactances connected between the anode and cathode of each of said tubes, one of said circuits including a capacity to tune the same to a frequency slightly above the desired mean frequency and the other of said circuits including a capacity to tune the same to a frequency slightly below the desired mean frequency, a circuit including a reactance connected between the grid and cathode of each of said tubes, cross' trodes, whereby a negative resistance eifect is produced in said tubes to insure the development of oscillations in the anode circuits of said tubes, the frequency of the oscillations produced being varied at signal, frequency in accordance with the variations in potential applied in phase 5 opposition 'to the screen grid electrodes of said tubes from said source.

10. A circuit for producing radio waves including, an oscillation generator tuned to oscillate at a frequency slightly above the normal 10 frequency of the oscillations to be produced, an oscillation generator tuned to oscillate at a frequency slightly below the frequency of the oscillations to be transmitted, thermionic coupling means between the output circuit of each of said 15 generators and the input circuit of the other of said generators, and means for varying the frequency of the oscillations produced by said combined oscillators between the mean frequencies of said separate oscillators as limits including 20 circuits for applying modulating potentials in phaseopposition to the impedances of said coupling means.

means for varying the frequency of the oscilla-- 35 tions generated by said combined oscillators between the frequencies of the separate oscillations as limits including a circuit for applying modulating potentials in phase opposition to said generators. 40

12. A device for producing carrier frequency oscillations including, a pair of thermionic tubes each having a cathode, an anode and a cxmtrol electrode, oscillation circuits connected between the anode and cathode of each of said'tubes, one 5 of said circuits including means to tune the same to a frequency above the desired mean frequency and the other of said circuits including means to tune the same to a frequency below the desired mean frequency, a tunedcircuit connected be- 50 tween the control grid and cathode of each of said tubes, a coupling tube having its input electrodes coupled to the anode of one of said pair of tubes and its output electrodes coupled to the control electrode of the other of said pair of tubes, a second coupling tube having its inputelectrodes coupled to the output electrodes of saidlast named tube of said pair of tubes, and its output electrodes coupled to the controlelectrode of said first named one of said pair of 50 tubes, whereby said tubes are connected in a completed cascade chain, a portion of which is tuned to oscillate at one frequency and a portion of which is tuned to oscillate at another frequency, the coupling between said portions 5 entraining the same to oscillate at a mean fre-' quency.-

13. A device as recited in claim 12 including. means for varying the electrical characteristic of said portions oppositely at signal frequency to 

